Re-transfer intermediate sheet for thermal transfer printing

ABSTRACT

A new re-transfer intermediate sheet comprising a supporting substrate having on one side an imageable layer and on the other a backcoat, is provided for thermal transfer printing of an article having a dye-receptive surface, by thermal retransfer. This method of printing comprise the steps of pressing together a dye-donor sheet and the imageable layer of the retransfer intermediate sheet, forming an image in the imageable layer by thermal transfer printing, pressing the thus-formed image-containing layer against the dye-receptive surface of the article, and applying heat to the intermediate sheet to effect retransfer of the image to the dye-receptive layer of the article. To improve protection against the physical conditions experienced in such retransfer process, the backcoat of the new intermediate sheet comprises a polymeric binder and a high loading of protective filler, preferably in amount of 100% to about 250% by weight of the binder.

This application is the national phase of international applicationPCT/GB97/01760 filed Jul. 3, 1997 which designated the U.S.

The invention relates to thermal transfer printing of an article byforming an image in an intermediate sheet by thermal transfer andthereafter thermally retransferring the image to a dye-receptive layeron the article; and in particular to the composition of the retransferintermediate sheet.

Thermal transfer printing is a process in which one or more thermallytransferable dyes are caused to transfer from selected areas of adye-donor sheet to a receiver by thermal stimuli, thereby to form animage. This is generally carried out in a printer having a thermal heador laser energy source, depending on the kind of dye-donor sheet used.Using a dye-donor sheet comprising a thin substrate supporting a dyecoatcontaining one or more uniformly spread dyes, printing is effected byheating selected discrete areas of the dye-donor sheet while the dyecoatis pressed against a dye-receptive surface of a receiver sheet, therebycausing dye to transfer to corresponding areas of the receiver. Theshape of the image transferred is determined by the number and locationsof the discrete areas which are subjected to heating. Full colour printscan be produced by printing with different coloured dyecoatssequentially in like manner, and the different coloured dyecoats areusually provided as discrete uniform panels arranged in a repeatedsequence along a ribbon-shaped dye-donor sheet.

In order to print articles other than flexible sheets, one method thatis commonly used is thermal retransfer. This is a two stage process,employing a retransfer intermediate sheet comprising a supportingsubstrate having a dye-receptive imageable layer on one side, usuallywith a backcoat on the other side to promote good transport through theinitial printer. In the first stage, an image is formed as above bypressing together a dye-donor sheet and the imageable layer of theintermediate sheet, and applying heat to selected positions of thedye-donor sheet to cause transfer of dye into that imageable layer,thereby to produce the image.

The image-containing intermediate sheet is then separated from thedye-donor sheet, and in the second stage of the process, is pressedagainst the article, with its image-containing layer contacting adye-receptive surface of the article. Heat is then applied to effecttransfer of the image, usually over the whole area of the imagesimultaneously and in a press shaped to accommodate the article.Alternatively with some appropriately shaped articles, heated rolls maybe used to provide the heat as the intermediate sheet and article arefed through. Thus although thermal retransfer techniques can be used forprinting laminar articles such as stiff cards, they are of particularapplicability to the printing of three dimensional articles such asmugs.

Not all of the dye which forms the image can retransfer to the articlein the thermal retransfer process, but the higher the proportion whichcan be caused to retransfer, the more intense will be the colours in theprinted article. The proportion which does retransfer depends on,amongst other things, the composition of the dye-receptive surface ofthe article. This may be the natural surface of that article where thelatter is formed of an appropriately dye-receptive material, but in mostinstances it is usual first to provide the article with a coating toform a surface of enhanced dye-receptivity.

A further factor influencing the degree of retransfer is the amount ofheat applied in the second, i.e. retransfer, stage. Heated pressesshaped according to the mug or other article to be printed, have beensold by a number of manufacturers, and typically these developtemperatures of 140-180° C. Under such conditions, the intermediatesheet can degrade, leaving debris in the press and ultimately stickingto the press when it is opened, causing defects to occur in theretransferred image. We have now developed a heat resistant backcoatcomposition to provide retransfer intermediate sheets with improvedprotection against the physical conditions experienced in suchretransfer presses.

Accordingly, one aspect of the invention provides a re-transferintermediate sheet for thermal transfer printing of an article bythermal retransfer, the intermediate sheet comprising a supportingsubstrate having on one side an imageable layer and on the other abackcoat, wherein the backcoat is a heat-resistant layer comprising apolymeric binder and a protective particulate filler in an amount of atleast 50% by weight of the binder.

According to a further aspect of the invention, a method of printing anarticle having a dye-receptive surface comprises the steps of pressingtogether a dye-donor sheet and an imageable layer of a retransferintermediate sheet comprising a supporting substrate having on one sidethe imageable layer and on the other a backcoat, forming an image in theimageable layer by thermal transfer printing, pressing the thus-formedimage-containing layer against the dye-receptive surface of the article,and applying heat to the intermediate sheet to effect retransfer of theimage to the dye-receptive layer of the article, characterised in thatthe backcoat is a heat-resistant layer comprising a polymeric binder anda protective particulate filler in an amount of at least 50% by weightof the binder.

The protective filler most suitably comprises mainly particles of 1-10μm mean diameter.

For the purpose of providing thermal resistance, the type of particle isless critical than the proportion used relative to the binder, althoughother properties may influence the optimum choice. We have used to goodeffect organic particles in the form of a poly(alkylsilylsesquioxane)compound, such as the methyl substituted compounds marketed in variousparticle sizes under the trade mark Tospearl, by Toshiba. Equallyeffective in providing heat resistance are inorganic particulates suchas hydrated alumina and the like. However the organic particles aregenerally preferred, in view of the more abrasive nature of compositionswith high loadings of hydrated alumina. Examples ofpoly(alkylsilylsesquioxane) particulate compounds available commerciallyinclude KMP-590 (Shinetsu Chemical); Tospearl 105, Tospearl 108,Tospearl 120, Tospearl 130, Tospearl 145 and Tospearl 240 (ToshibaSilicone); and Torefil R-925 and Torefil 930 (Toray Dow Corning).

Compared with retransfer intermediate sheets with backcoats containingonly small quantities of particles, typically about 1-10% by weight ofthe binder in the past, we find we obtain a noticeable improvement inheat resistance with as little as 50% by weight of the binder. Howeverwe prefer to use at least 100%, especially at least 200% by weight ofthe binder, as the improvement in heat resistance increases withincreased loading. At higher loadings, other properties of the backcoatcan deteriorate, e.g. to become less readily coatable as a compositionduring manufacture, or more brittle once dried as a coating, but thisdepends on the resin used for the binder and on the nature of theparticles selected. Some of these difficulties with high filler loadingscan be mitigated by incorporating other additives into the composition.For example, in the preferred embodiments using particles at loadings ofabout 200% by weight of the binder or above, we prefer to include ametal phosphate salt of stearic acid in an amount of from 1 to 20% ofthe binder, to stabilise the solution and improve manufacturability. Itmay similarly be added to compositions containing lower particleloadings, but the lower the loading levels, the less is this of benefit.Subject to the above limitations, our preferred range for the amount ofprotective filler is generally from 100% to about 250% by weight of thebinder.

Where other particles are also incorporated, it may be necessary to useless of the protective 1-10 μm particles than the maximum quantity thatcould otherwise be used. Examples of such other particles which mayusefully be added include slightly larger particles added as ananti-blocking agent to improve handling. Our preferred anti-blockingagent comprises particles of 8-15 μm mean diameter, in an amount of10-25% by weight of the binder.

Suitable binders include cellulosic resins, such as cellulose acetateproprionate and cellulose acetate butyrate. The binder need not becross-linked in order to benefit in terms of heat resistance from thepresent high loadings of particulates. However, we generally do preferto provide some degree of cross-linking by the addition of small amountsof crosslinking agent. The cellulose resins can be crosslinked byisocyanates or by melamine cross linking agents in acid conditions.

The dried backcoat of the invention is preferably within a thicknessrange of 1-10 μm, especially 1-5 μm, as thicker backcoats provide littleextra protection, and lead to lower versatility, especially during thefirst, image-forming, stage.

The supporting substrate may typically be paper, especiallypolyolefin-coated paper. This is a support material which provides avery good quality of retransferred image, but which was particularlyprone to heat induced problems during retransfer prior to the protectionafforded by the present backcoats.

The invention is illustrated by the following Examples.

EXAMPLE 1

A backcoat composition A was prepared, and coated onto a substrate ofpolyethylene coated paper, then dried to give a coating of thickness 3μm. An imageable layer had previously been coated onto the other side ofthe polyethylene coated paper, to complete a retransfer intermediatesheet according to the invention.

backcoat composition A cellulose acetate proprionate (CAP) 31.45 wt %(482-0.5 - from Eastman Kodak) Atmer 190 (antistatic agent) 0.5 wt %Beetle 692 (melamine cross-linker) 1.66 wt % p-toluene sulphonic acidcatalyst 0.33 wt % Tegomer 2311 (silicone levelling agent) 0.03 wt %Tospearl 120 (particles - 2.0 μm mean diameter) 62.89 wt % calciumstearyl phosphate (stabiliser) 3.14 wt %

(Atmer is a trade mark of ICI, Beetle 692 is a trade mark of BritishIndustrial Plastics, Tegomer 2311 is a trade mark of Goldschmidt andTospearl is a trade mark of Toshiba.)

In this composition the amount of the particulate Tospearl isapproximately 200% by volume of the binder (CAP).

EXAMPLES 2-4

Three further intermediate sheets were prepared according to theinvention, using corresponding compositions but wherein the fillercontents were changed as follows:

Composition B Tospearl 120 at 100% by weight of the binder

Composition C Tospearl 120 at 50% by weight of the binder

Composition D Apyral at 100% by weight of the binder

(Apyral is a trade mark of QLT in respect of hydrated alumina filler)

Test results

To evaluate the retransfer intermediate sheets prepared in Examples 1-4above, they were used to print mugs in a standard mug-printing press,and as a control, a previous retransfer sheet containing particles butonly in an amount of approximately 1% by weight of the binder, wassimilarly tested.

Each intermediate sheet was printed with blocks of colour, in a thermaltransfer printer whose heat source was a thermal head having a row ofprogrammable pixel heaters, in normal manner. Using a mug-shapedheatable press, each imaged intermediate sheet was placed in the presstogether with a mug precoated with a resin to give a dye-receptivesurface against which was placed the imaged layer of the intermediatesheet. The press was then activated to apply heat and pressure to theback of the intermediate sheet, to thermally retransfer dyes from theintermediate sheet into the dye-receptive surface layer of the mug. Atthe end of each retransfer process, the press was opened, and theprinted mug removed. Both the press and the image retransferred into themug were examined, and the results were as shown in the table below,

BACKCOAT COMPOSITION RESULT Composition A + + Composition B +Composition C o Composition D + + Control sheet − −

where:

xx=excellent

x=very good

o=OK but room for improvement

--=unsatisfactory

EXAMPLE 5

A retransfer intermediate sheet according to the invention was preparedessentially as described in Example 1, except that a anti-blocking agentwas also added to the backcoat composition (composition E) to improvehandling.

backcoat composition E cellulose acetate proprionate (CAP) 100 parts byweight (482-0.5 - from Eastman Kodak) Tospearl 120 200 parts by weight(protective particles - 2.0 μm mean diameter) Pergopak 17 parts byweight (anti-block particles - 8-15 μm mean diameter) calcium stearylphosphate (stabiliser) 10 parts by weight Beetle 692 (melaminecross-linker) 5.0 parts by weight p-toluene sulphonic acid catalyst 1.0parts by weight Atmer 190 (antistatic agent) 1.5 parts by weight Tegomer2311 (silicone levelling agent) 0.1 parts by weight

(Pergopak is a trade mark of Martinswerk)

The retransfer intermediate sheet thus prepared was used to print mugsin a standard mug-printing press, essentially as described for Examples1-4. Handling during printing was excellent. When the press was openedand the printed mug removed after the retransfer process, both the pressand the image retransferred into the mug were examined and found to bein excellent condition, with no significant degradation of theretransfer sheet being apparent.

What is claimed is:
 1. A re-transfer intermediate sheet for thermaltransfer printing of an article by thermal retransfer, comprising asupporting substrate having on one side an imageable layer and on theother a backcoat, wherein the backcoat is a heat-resistant layercomprising a polymeric binder and protective particulate filler,characterized in that the protective particulate filler is included inan amount of at least 100% by weight of the binder, the amount of fillerbeing such that the backcoat is rendered resistant to degradation byheat applied thereto when said sheet issued in thermal re-transferprinting of an article.
 2. A retransfer intermediate sheet according toclaim 1, wherein the protective filler mainly comprises particles of1-10 μm mean diameter.
 3. A retransfer intermediate sheet according toclaim 2, wherein the protective filler mainly comprises organicparticles.
 4. A retransfer intermediate sheet according to claim 3,wherein the organic particles comprise a poly(alkylsilylsesquioxane). 5.A retransfer intermediate sheet according to claim 2, wherein theprotective filler mainly comprises inorganic particles.
 6. A retransferintermediate sheet according to claim 2, wherein the backcoatcomposition contains the protective filler in an amount of at leastabout 200% by weight of the binder.
 7. A retransfer intermediate sheetaccording to claim 2, wherein the backcoat composition contains theprotective filler in an amount of 100% to about 250% by weight of thebinder.
 8. A retransfer intermediate sheet according to claim 1, whereinthe heat-resistant layer contains an anti-blocking agent comprisingparticles of 8-15 μm mean diameter, in an amount of 10-25% by weight ofthe binder.
 9. A retransfer intermediate sheet according to claim 1,wherein the backcoat composition contains a metal phosphate salt ofstearic acid in an amount of from 1 to 20% by weight of the binder. 10.A retransfer intermediate sheet according to claim 1, wherein the binderis a cellulosic resin.
 11. A retransfer intermediate sheet according toclaim 1, wherein the polymeric binder is cross-linked.
 12. A retransferintermediate sheet according to claim 11, wherein the protective fillercomprises poly(methylsilysesquioxane) particles of 1-10 μm meandiameter.
 13. A method of printing an article having a dye-receptivesurface comprising the steps of pressing together a dye-donor sheet andan imageable layer of a retransfer intermediate sheet comprising asupporting substrate having on one side the imageable layer and on theother a backcoat, forming an image in the imageable layer by thermaltransfer printing, pressing the thus-formed image-containing layeragainst the dye-receptive surface of the article, and applying heat tothe intermediate sheet to effect retransfer of the image to thedye-receptive layer of the article, wherein the backcoat is a heatresistant layer comprising a polymeric binder and a protectiveparticular filler, characterized in that the protective particulatefiller is included in an amount of at least 100% by weight of thebinder, the amount of said filler being such as to render said backcoatresistant to degradation by the heat applied to effect the retransfer.14. A retransfer intermediate sheet for thermal transfer printing of anarticle by thermal retransfer, comprising a supporting substrate havingon one side an imageable layer and on the other a backcoat, wherein thebackcoat is a heat-resistant layer comprising a polymeric binder and aprotective particulate filler, characterized in that the protectiveparticulate filler is selected from poly(alkylsilylsesquioxane)compounds and hydrated alumina fillers in an amount of at least 100% byweight of the binder.